This question involves equilibrium analysis for objects moving at constant velocity through fluids. The rock experiences weight mg downward, plus buoyant force and drag both upward. Since the rock moves at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the sum of upward forces (buoyant force plus drag) must equal the downward weight mg. Choice A incorrectly assumes downward motion requires downward net force, but constant velocity requires zero net force regardless of motion direction.

This question tests net force analysis when objects accelerate downward in fluids. The object experiences weight mg downward and buoyant force upward, with negligible drag. Since the object has increasing downward speed, its acceleration is downward. By Newton's second law, downward acceleration requires downward net force, meaning the weight must exceed the buoyant force. Choice A incorrectly assumes fluids create upward net force, but acceleration direction determines net force direction regardless of the fluid medium.

This question tests net force direction when objects decelerate while moving upward in fluids. The object experiences weight mg downward and buoyant force upward only. Since the object moves upward but slows down, its acceleration is downward (opposite to velocity direction). By Newton's second law, downward acceleration requires downward net force, meaning weight exceeds buoyant force. Choice D incorrectly assumes upward motion implies balanced forces, but deceleration requires net force opposing motion direction.

This question tests net force direction analysis when objects accelerate in fluids. The cube experiences weight mg downward and buoyant force upward, with negligible drag. Since the cube falls through water and speeds up downward, its acceleration is downward. By Newton's second law, downward acceleration requires downward net force, meaning weight exceeds buoyant force. Choice C incorrectly assumes net force could be zero despite changing speed, but acceleration requires non-zero net force in the direction of acceleration.

This question examines force relationships for objects moving at constant velocity upward in fluids. The ball experiences buoyant force upward, weight mg downward, and drag downward. Since the ball moves upward at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the upward buoyant force must equal the sum of downward forces: FB = mg + FD. Choice A incorrectly sets buoyant force equal to weight only, ignoring the additional downward drag force that must also be balanced.

This question examines net force direction for objects accelerating downward in fluids. The object experiences buoyant force upward and weight mg downward only. Since the object moves downward while speeding up, its acceleration is downward. By Newton's second law, downward acceleration requires downward net force, meaning the weight exceeds the buoyant force. Choice D incorrectly assumes having only two forces guarantees zero net force, but net force depends on the relative magnitudes of opposing forces.

This question involves equilibrium analysis for light objects held submerged by external forces. The object experiences buoyant force upward, weight mg downward, and tension downward while at rest. Since the object is in equilibrium, Newton's first law requires zero net force, so the upward buoyant force must equal the sum of downward forces. Therefore, the buoyant force equals mg + T, with buoyant force balancing both weight and downward tension. Choice A incorrectly assumes buoyant force equals weight only, ignoring the additional downward tension.

This question tests equilibrium analysis for objects moving at constant velocity in fluids. The object experiences buoyant force upward, weight mg downward, and drag downward. Since the object moves upward at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, all forces must balance with upward forces equaling downward forces. Choice A incorrectly assumes upward motion requires upward net force, but constant velocity specifically requires zero net force regardless of motion direction.

This question tests equilibrium analysis for objects held stationary by external forces in fluids. The rock experiences buoyant force upward, tension upward, and weight mg downward while at rest. Since the rock is in equilibrium, Newton's first law requires zero net force, so upward forces must equal downward forces. Therefore, FB + T = mg, with both upward forces combining to balance the weight. Choice A incorrectly assumes multiple upward forces automatically create upward net force, but equilibrium requires force balance regardless of force directions.

This question involves equilibrium analysis for neutrally buoyant objects after external impulses. The sphere experiences buoyant force upward and weight mg downward, with negligible drag. Since the sphere moves upward at constant speed after the push, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the buoyant force must equal mg for force balance. Choice A incorrectly assumes upward motion requires upward net force, but constant velocity motion requires zero net force.